Singlet oxygen in biochar-based catalysts-activated persulfate process: From generation to detection and selectivity removing emerging contaminants

Emerging contaminants (ECs) have become a pressing environmental concern. The biochar-based catalysts (BCs)-activated persulfate (PS) process, which employs singlet oxygen (1O2) and effectively mitigates the impact of complex environmental backgrounds while enhancing mass transfer, is garnering growing attention in ECs removal. However, challenges persist that impede further advancement of this method. This review delves into the inner workings of BCs, focusing on their oxygen functional group (OFG), defect structure, and persistent free radicals (PFRs) which activate PS to generate 1O2, by examining their electron structure. Additionally, heteroatom doping bolsters 1O2 generation by boosting the electronic conductivity of BCs and creating additional reactive sites. The generation of 1O2 in BCs/PS processes is mainly attributed to SO5•- and O2•. Then, methods to detect 1O2, including quenching experiments, electron paramagnetic resonance (EPR) spectra, solvent exchange, and probe method are reviewed, highlighting the efficacy of multiple methods in mitigating the risk of false positives typical of a single method. Additionally, Density Functional Theory (DFT) calculations are used to predict the reactivity of 1O2 with ECs like per- and polyfluoroalklyl substances (PFAS). Finally, further research directions are also pointed to enhance the development of 1O2-AOPs for ECs removal.